Normally, as the tempering progresses, the hardness and the strength decrease, but the ductility and impact strength increase. But in certain steels, there is an unexpected decrease of the impact strength in certain ranges of temperatures as schematically illustrated in Fig. 7.13.
This indicates that there are two main types of embrittlement during tempering:
1. Tempered martensite embrittlement (TME),
2. Temper embrittlement (TE).
Both these embrittlements raise the impact transition temperature (ITT) to higher temperatures. Fig. 7.14 indicates the increase in impact transition temperature, Δ (ITT) due to TE in SAE 3140 steel. Normally, the degree of both type of embrittlements is expressed in terms of relative displacement of ITT, i.e., by Δ (ITT) as illustrated in Fig. 7.14. In both, the transgranular fracture mode is replaced by an intergranular (IG) mode below the transition temperature, i.e., this show bright intercrystalline fracture along original austenite grain boundaries.
Inspite of these similar effects, the two types of embrittlements are two separate phenomena, which occur in two different temperature ranges. Moreover, TME is a much faster process usually occurring in one hour, whereas TE takes many hours. Temper embrittlement is of much greater concern from practical point of view, as the rotors and shafts of power generating equipment even after tempering above 600°C, develop it when thick sections cool very slowly through the range (450°-600°C).
As TME develops after tempering in range 260°C to 370°C, it is also called as, “350°C” embrittlement, or 500°F embrittlement. It is called ‘one-step embrittlement’ as during heating only in this range, TME develops. It is also called ‘irreversible embrittlement because a steel embrittled by tempering in this range, if heated further to above 400°C (above the critical range), becomes tougher, and the tempered martensite embrittlement does not occur again if cooled down to, or tempered in the range of 260°C to 370°C again.
All steels, including the plain carbon steels are prone to irreversible embrittlement to some extent, and that is why tempering range of 260°-370°C is avoided in all steels, though it is a malady of low alloy steels.
The embrittlement is associated with the change in the structure of carbide from epsilon (e) to cementite in the form of a film at the grain boundaries. On tempering at higher temperatures, this film disappears and cannot be restored, on repeated heating in 260°-370°C temperature range.
Although, tempered martensite embrittlement is concurrent with the precipitation of cementite, but such precipitation is not in itself the cause of loss of impact toughness, as the embrittlement does not occur if P, Sb, Sn, As, or N are not present in steel. Addition of sufficient silicon to the steel inhibits the formation of cementite in the critical range, as silicon dissolved in epsilon carbide, increases its stability, and thus, embrittlement does not occur.
The temperature range is too low to attribute embrittlement to the diffusing metalloids such as Sb to the grain boundaries of austenite. It is more likely that smaller and more mobile atoms of P, or N can cause it, i.e., the process may start with slight segregation of P to austenite grain boundaries, perhaps promoted by Mn, or P may segregate to austenite grain boundaries during austenitisation, and is present there, even after quenching and tempering to the critical range.
P weakens the adhesion at the austenite grain boundaries. The subsequent formation of cementite (which occurs in the critical range) at the original austenite grain boundaries, may increase the segregation of impurity atoms as these are rejected from the growing cementite particles. Thus, impurity segregation of P or N, etc. weakens the adhesion of original austenite grain boundaries, and cracking of the cementite initiates inter-granular fracture (IG).
2. Temper Embrittlement:
This sickness of alloy steels occurs when they are tempered in the range 450°C to 600°C. It is also called reversible embrittlement (as well as two-step embrittlement), because it occurs, when steels are tempered in this range, but gets removed, when heated to high temperatures, but occurs again on slow continuous cooling through this range from that high temperature (> 600°C).
The degree of embrittlement depends on the rate of cooling in the range 600°- 450°C as illustrated below:
The phenomenon of temper embrittlement results in loss of toughness as measured by notched impact test (without affecting very much the hardness, yield strength, ultimate tensile strength, elongation and fatigue properties), and a rise in ductile-to brittle transition temperature occurs (presence of 0.04% Sb raises the transition temperature from -40°C to 600°C of 0.3% carbon Cr-Ni steel), with an intergranular (IG) fracture below the transition temperature along the original austenitic grain boundaries.
No precipitate is observed metallographically. Most of the reagents did not reveal any difference. However, an ethereal solution of picric acid in which zephiran chloride was added, preferentially attacks the grain boundaries in the embrittled state, and not in the tough state.
Carbon steels in general, but with less than 0.5% Mn do not show temper embrittlement. Alloy steels of high purity do not show it. It is caused primarily by Sb and P and secondarily by Sn or As (even in amounts as low as 0.01%) in presence of elements like Ni, Mn, Cr, Si in steels. The highest effect is in Ni-Cr and Mn-Cr steels. Presence of elements like Mo, Ti, Zr delay, or remove embrittlement.
The characteristic features of temper embrittlement are best explained by the concept of double equilibrium segregation (co-segregation). The impurity solutes are the surface active elements in iron, i.e., these reduce the grain boundary energy, and thus reduce the cohesion. Elements like Sb, P, As, Sn interact with certain elements like Ni and Mn in steels. These interactions lead to co-segregation of alloying elements and the impurity elements such as between Ni-Sb, Ni-P, Ni-Sn and Mn-Sb, as confirmed by Auger spectroscopy.
The reason of co-segregation is the stronger interaction between them than, between either of these and iron. If the interaction is very strong then, co-segregation does not occur, but a scavenging effect is got, as happens between Mo-P, Ti-P, which is the cause of elimination of embrittlement by 0.5% Mo in such steels.
If larger amount of Mo, Ti, Zr are present, then these elements slowly react with carbon to form stable carbides releasing the impurity atoms to segregate to the boundaries. Solute atoms segregate to original austenite grain boundaries as these are the large 650 angle boundaries, where accommodation is easier. Elements like Cr promote co-segregation of Ni and P, and also Ni and Sb.
In these steels, grain boundaries are also the sites for carbide precipitation (either cementite, or alloy carbides), which provides the sites for IG crack nucleation as the adhesion has already become less. Increasing the grain size of austenite increases embrittlement, because the size of the dislocation array is larger here, and the higher stress thus helps to nucleate easily the crack at the carbide.
In plain carbon steels, even when impurity atoms are present, co-segregation is missing, so that equilibrium concentration of impurity atoms is low, and not enough to cause severe embrittlement. In alloy steels of high purity, the absence of impurity atoms again does not cause co-segregation, but in commercial alloy steels, mutual promotion of segregation of impurities and the alloying elements takes place due to strong interaction between them.
Additional segregation may take place, when two alloying elements are present simultaneously, such as Ni and Cr. At high temperatures ( > 600°C), thermal vibrations make the equilibrium segregation low enough not to cause embrittlement, and at lower temperature (< 450°C), the diffusion of the elements is too low to cause enough co-segregation with in the normal tempering time. Fig. 7.15 illustrates effect of grain boundary segregation of impurities (P, Sb, Sn) on the transition temperature of Ni-Cr steel of constant hardness and grain size. Sb is very effective in raising the transition temperature.
During tempering, the amount of segregation of impurities at a given temperature is a function of temperature and time of tempering at that temperature as well as the diffusivity of the impurity. At high temperatures (> 600°C), the equilibrium segregation is low, not enough to cause embrittlement, whereas at low temperatures, the diffusivity of impurities is too low to cause enough segregation in normal tempering time. That is why, TE is observed in range 450°C to 600°C.
Fig. 7.16 illustrates time-temperature relationship to develop temper embrittlement to get constant transition temperature. The iso-embrittling curve is C-shaped. The amount of embrittlement increases with time at a temperature to a maximum, and then decreases. The rate of embrittlement increases with tempering temperature and time to attain a maximum, and then decreases. Fastest embrittlement occurs at 550°C.
The following means are normally recommended to minimise the effects of temper embrittlement:
1. Keep the impurities such as Sb, P, Sn, As as low as possible.
2. Alloy the steel with Mo (0.5 – 0.75%).
3. Quench from tempering at higher temperatures (> 600°C).